Despite being freely prescribed across the US, a new study shows that sleeping tablets increase the risk of death for those taking them—and are associated with up to 500,000 extra fatalities a year in the US alone. And nobody knows why.

A report published in the BMJ Open journal explains that common sleeping pills—like Restoril, Normison, Ambien and Stilnox—are associated with a four-fold increase in the risk of death. The study, carried out by the Scripps Clinic Viterbi Family Sleep Center in La Jolla, California, compared death rates among more than 10,500 people who take sleeping pills with 23,600 others that do not, adjusting in each case for age, health and other factors.

Risk increased with the number of doses taken, and those taking 132 doses or more a year faced a five times greater risk of death. They were also 35 per cent more likely to be diagnosed with cancer.

It's worth noting, however, that the study only shows correlation between sleeping pill use and risk of death. To date, there's never been a properly coordinated, large-scale randomized clinical trial to investigate whether sleeping pills do pose this kind of risk—and that's what's needed to determine if the link is causal.

Still, the figures are dramatic and certainly cause for concern. In the meantime, it's worth remembering that sleeping pills should only be used when insomnia is severe or debilitating, and even then only taken for short periods less than four weeks in duration. [BMJ Open via The Guardian; Image: Dominique Godbout]